Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Olefin gas cracker furnaces are normally designed to crack ethane, propane and on occasion butane, but typically lack the flexibility to crack heavier liquid feedstocks, particularly those that produce tar in amounts greater than one percent. As gas feeds tend to produce little tar, primary, secondary, and even tertiary transfer line exchangers (TLEs) are utilized to recover energy through the generation of high pressure and medium pressure steam, as the furnace effluent cools from the furnace outlet to the quench tower inlet. The process effluent is normally then fed to a quench tower wherein the process effluent is further cooled by direct contacting with quench water.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipe still bottoms, and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile components of these feedstocks may lay down as coke in the convection section of conventional pyrolysis furnaces. Only low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Cracking heavier feedsthat contain non-volatiles causes convection section coking, often requiring costly shutdowns for cleaning.
Gas and steam crackers designed to operate on gaseous feedstocks, while limited in feedstock flexibility, require significantly lower investment when compared to liquid feed crackers designed for naphtha and/or heavy feedstocks that produce higher amounts of tar and byproducts. However, as may be appreciated, when the price of natural gas price is high relative to crude, gas cracking tends to be disadvantaged when compared with the cracking of virgin crudes and/or condensates, or the distilled liquid products from those feeds. (e.g., naphtha, kerosene, field natural gasoline, etc). In such an economic environment, it would be desirable to extend the range of useful feedstocks for gas fed crackers to include liquid feedstocks that contain higher levels of non-volatiles.
Advantaged steam cracking feeds frequently contain asphaltenes, which laydown as coke in the convection section of conventional pyrolysis furnaces. Contaminated condensates and full range virgin gas oils (FRVGO) with up to 400 ppm asphaltenes are typical of such advantaged feeds. However, feeds with greater than 100 ppm asphaltenes cause the thickness of the coke layer to increase rapidly in part because the coke produced by the asphaltenes typically is found within a few rows of the heat exchange tubes of the convection section. Since pressure drop is a strong function of tubing diameter, a fast growing coke layer causes the convection section pressure drop to increase rapidly. For example, a one-half inch layer of coke in a five inch diameter tube triples the pressure drop across the tube, while the same one-half inch layer of coke in a three inch diameter tube increases the pressure drop by about nine times. As such, it would be desirable to reduce the rate of increase in pressure drop across a furnace convection section to enable the use of advantaged steam cracking feeds while extending the run time between cleanings.
U.S. Pat. No. 7,090,765 proposes a process for cracking hydrocarbon feed with water substitution, the process including the steps of heating hydrocarbon feed, adding water to the heated feed, adding dilution steam to the heated feed to form a mixture, heating the resulting mixture and feeding the resulting heated mixture to the furnace, wherein the water is added in an amount of from at least about 1% to 100% based on water and dilution steam by weight. This process proposed includes a knockout pot outside the convection section that allows non volatile asphaltenes to be removed to avoid convection section fouling.
U.S. Patent Publication No. 2005/0261532 proposes a process and apparatus for removing coke formed during steam cracking of hydrocarbon feedstocks containing resides. Steam is added to the feedstock to form a mixture which is thereafter separated into a vapor phase and a liquid phase by flashing in a flash/separation vessel, separating and cracking the vapor phase, and recovering cracked product. Coking of internal surfaces in and proximally downstream of the vessel is said to be controlled by interrupting the feed flow, purging the vessel with steam, introducing an air/steam mixture to at least partially combust the coke, and resuming the feed flow when sufficient coke has been removed.
U.S. Patent Publication No. 2006/0249428 proposes a process for steam cracking heavy hydrocarbon feedstocks containing non-volatile hydrocarbons. The process includes the steps of heating the heavy hydrocarbon feedstock, mixing the heavy hydrocarbon feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase, and varying the amount of the fluid and/or the primary dilution steam stream mixed with the heavy hydrocarbon feedstock in accordance with at least one selected operating parameter of the process, such as the temperature of the flash stream before entering the flash drum.
Despite these advances in the art, there is a need for a method of reducing the rate of increase in pressure drop across a furnace convection section to enable the use of advantaged steam cracking feeds.